The present invention relates to a compact zoom lens system having a high zoom ratio and a wide view angle. More particularly, the present invention relates to a wide-angle zoom lens system which is mainly used with a compact camera or the like having a relatively short back focus and which has a view angle of 70.degree. or more at the wide end and a zoom ratio of 2.5 or higher.
There have heretofore been proposed a large number of zoom lens systems for compact cameras of a two-unit type in which a positive lens unit and a negative lens unit are disposed in order from the object side, as disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 56-128911, 62-264019, 63-266413, and so forth. However, in most of these zoom lens systems, the view angle at the wide end is about 60.degree. at most.
To meet the demand for compact cameras having a wider view angle, some zoom lens systems for compact cameras which are of the two-unit type and which provide a view angle of 70.degree. or more at the wide end have recently been proposed. For example, see Japanese Patent Application Laid-Open (KOKAI) Nos. 02-284109, 03-200913, and 04-22911.
However, none of these known zoom lens systems are satisfactory in terms of performance. That is, the conventional zoom lens systems suffer from some problems.
For example, the curvature of field largely bends toward the positive side at the wide end, and a large distortion toward the positive side also occurs at the wide end. In addition, there are large fluctuations in the axial and lateral chromatic aberrations due to zooming.
As zoom lens systems having a view angle of 70.degree. or more at the wide end and a zoom ratio of 2.5 or more, there is known a three-unit type zoom lens system having, in order from the object side, a negative lens unit, a positive lens unit, and a negative lens unit, as disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 02-37317 and 02-201409, and another three-unit type zoom lens system having, in order from the object side, a positive lens unit, a positive lens unit, and a negative lens unit, as disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 02-135312.
These three-unit type zoom lens systems, however, have the problem that since the overall length is long and the outer diameter of the first lens unit is also large, the overall size of the camera is correspondingly large. Particularly, at the wide end, the visual field of the viewfinder is vignetted by the lens frame because of the long overall length of the lens system.
If it is intended to improve a two-unit type zoom lens system having a positive lens unit and a negative lens unit so that the zoom ratio is raised to 2 or more and the overall length is minimized, the curvature of field bows toward the negative side at the middle focal length position, and it bows toward the positive side at the tele end.
To solve this problem, Japanese Patent Application Laid-Open (KOKAI) Nos. 02-34812 and 01-307715 divide the 1-st lens unit into a positive or negative lens unit and a positive lens unit and employ a device for non-linearly varying the spacing between these two lens units.
In these known zoom lens systems, however, the view angle at the wide end is about 60.degree., and the zoom ratio is about 2.3.
Japanese Patent Application Laid-Open (KOKAI) No. 64-88512 discloses a zoom lens system capable of attaining a zoom ratio of 2.5 or higher with a lens arrangement having four lens units, that is, a negative lens unit, a positive lens unit, a positive lens unit, and a negative lens unit, in the same way as in the case of the present invention, described later. In this zoom lens system, however, the view angle at the wide end is about 62.degree., and since each lens unit moves individually, a large number of cams are needed for zooming, which is unfavorable from the viewpoint of designing a lens frame for the zoom lens system.
Japanese Patent Application Laid-Open (KOKAI) No. 62-235916 also discloses a zoom lens system having four lens units, i.e., a negative lens unit, a positive lens unit, a positive lens unit, and a negative lens unit, in which the first and third lens units are adapted to move together as one unit in the same way as in the present invention. In the zoom lens system of this publication, however, the second lens unit is adapted to move for the purpose of correcting lateral chromatic aberration and hence monotonously moves toward the first lens unit. Thus, the second lens unit does not contribute to the correction of the bowing of the field curvature at the middle focal length position, which is a problem arising when the zoom ratio is raised, as described above. Therefore, the zoom ratio is only about 2, and the view angle at the wide end is only about 62.degree..
As has been described above, to meet the demand for compact cameras having a wider view angle and a higher zoom ratio, the view angle at the wide end has been widened to 70.degree. or more and the zoom ratio has also been raised to 2 or more in recent years. If it is intended to widen the view angle at the wide end by using a conventional two-unit type zoom lens system having a positive lens unit and a negative lens unit, the curvature of field largely bends toward the positive side at the wide end, and a large distortion toward the positive side also occurs at the wide end. If it is intended to further raise the zoom ratio, the curvature of field bows toward the negative side at the middle focal length position, and it bows toward the positive side at the tele end. In addition, there are large fluctuations in the axial and lateral chromatic aberrations due to zooming.